2,3,3,3-tetrafluoropropene (HFO-1234yf) represented by chemical formula: CF3CF═CH2 is considered to be promising as a refrigerant for car air-conditioners because its global warming potential (GWP) is low.
In regard to a method for producing HFO-1234yf, there are various known methods such as a method in which CCl3CF2CH3 as a starting material is reacted with hydrogen fluoride (HF) having an amount exceeding the stoichiometric amount (PTL 1); a method in which fluorocarbon represented by CF3CFHCFH2 is subjected to dehydrofluorination (PTL 2), etc.
In these methods, outflow from the reaction vessel is a mixture containing not only a desired product, i.e., HFO-1234yf, but also HF in an amount equal to or greater than the equimolar amount of HFO-1234yf. Accordingly, HF needs to be removed from the mixture containing HFO-1234yf and HF in order to purify and commercialize HFO-1234yf. As a method therefor, there is a known method in which a mixture containing HFO-1234yf and HF is treated with water or an alkali solution to adsorb HF. However, this method requires a large amount of water or alkali solution, resulting in the discharge of a large amount of industrial wastewater. It is thus undesirable in terms of environmental protection and production costs.
Further, as another method for removing HF, there is a method in which a mixture containing HFO-1234yf and HF is reacted with H2SO4 so as to collect HF as hydrofluoric-sulfuric acid. However, in this method, the produced hydrofluoric-sulfuric acid has strong corrosivity, and thus materials for the equipment used must be highly corrosion resistant. This leads to an increase in production costs.
Further, in the case of the above method for removing HF, high technology is required to reuse the removed HF. This leads to an increase in production costs not only when HF is discarded but also when the collected HF is recycled.
As a method for solving these problems, for example, PTL 3 discloses a method for separating HFO-1234yf from HF contained in a mixture of HFO-1234yf and HF by using, as an extractant, a compound having a high mutual solubility with HFO-1234yf. However, in this method, a step of separating HFO-1234yf and the extractant is necessary after removing HF. This contributes to an increase in production costs. Further, use of an extractant creates a risk of contamination by impurities that are unnecessary for the process, and thus adds an increased burden in terms of process control and quality control.
Further, for example, PTL 4 discloses a method for obtaining HFO-1234yf from the bottom of a distillation column by distilling a mixture of HFO-1234yf and HF, and withdrawing an azeotrope-like mixture of HFO-1234yf and HF from the top of the distillation column. However, this method requires withdrawal of a large amount of HFO-1234yf along with HF from the top of the column, and thus the size of the distillation column is relatively large. Also, when a method for recycling an azeotropic mixture is employed, the size of the equipment used for the process is relatively large because the amount of a mixture of HFO-1234yf and HF to be circulated is large. These are the factors that contribute to an increase in equipment costs and the running costs of the process.